WOOD IN GENERAL 247 
responds to the bony skeleton of animals in contrast with 
their tendons and hairs to which we may liken internal and 
external vegetable fibers respectively. 
A definition of wood in the economic sense requires that 
it be distinguished principally from fiber, because of the 
especially close similarity between them. Fibers, we have 
seen, are sometimes woody, while all true woods, as will 
presently appear, are fibrous. Cellulose is the main con- 
stituent of each. Woods and woody fibers contain in addi- 
tion to cellulose more or less of a substance (or mixture of 
substances) known as lignin. This is of uncertain chemical 
composition though known to consist of the same elements 
as cellulose. Like that substance it permits water and gases 
to pass readily through it. It is distinguished from cellulose 
by turning yellow instead of blue when treated with sul- 
-phuric acid and iodine. It is the fact that wood is used in 
comparatively large, firm masses which chiefly distinguishes 
it from fibers; while it is the fibrousness of wood that most 
readily distinguishes it from cork and other massive materials 
to be presently studied. Let us then for our present purpose 
define wood as the comparatively hard mass of fibrous ma- 
terial which serves mainly for mechanical support in plants 
and in various artificial structures. 
From earliest times wood has been the most widely useful 
material of construction. Our civilization has been developed 
largely upon its possibilities. In prehistoric times wherever 
it was abundant, wood was used almost exclusively for build- 
ings, utensils, and implements; though in regions less favor- 
ably situated various substitutes of course had to be found. 
Even before skill in metal-working had been acquired men 
were able to shape wood by means of their rude stone tools 
into many highly useful forms. Thus, only the rudest means 
are necessary for making from a single log a “dugout” canoe 
capable of holding many men: a fire kept alive along the top 
of a fallen trunk burns or chars the wood so that it may be 
scraped away till the desired form is reached. With the 
coming of metal tools and their improvement from time to 
time, more extensive use could be made not only of wood, 
but also, and for the same reason, of stone and other hard 
